1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to such a system for maximizing the useful data transmission throughput in a data call in which data is transmitted between wireless stations on multiple assigned channels.
2. Related Art
A wireless communication system can be used to transmit synchronous and asynchronous packet data between a wireless transmitter and a wireless receiver. For example, the wireless communication system can operate in accordance with a High Speed Packet Data (HSPD) feature of the “TIA/EIA/IS-95B Mobile Station-Base Station Compatibility Standard for Dual-mode Wideband Spread Spectrum Cellular Systems” (hereinafter referred to as IS-95B) to achieve a packet data transmission bandwidth of up to 115 kilobits-per-second (kbps). Under IS-95B, a mobile station can transmit data to a base station receiver on an IS-95B reverse-link traffic channel including a fundamental code channel (FCCH) and up to seven additional Supplemental Code Channels (SCCHs). The FCCH is a variable rate channel capable of operating at data transmission rates including a full rate, a half rate, a quarter rate, and an eighth rate. On the other hand, the SCCH operates only at a full rate when data is to be transmitted, and at a zero rate when no data is available.
Packet data transmitted on the FCCH and SCCHs is partitioned into 20 millisecond (ms) variable rate data frames. Although the data rate can change rapidly, for example, on a frame by frame basis, rate information is typically not included in each transmitted data frame for at least two reasons. First, including rate information in each data frame wastes data bandwidth, and second, corruption of such transmitted rate information would adversely affect the entire frame. Since rate information is not included in each transmitted data frame, the receiver must determine from each received data frame (without the aid of embedded rate information) the rate at which the frame was transmitted, to thereby enable the receiver to properly process the data in the data frame. Known methods of determining data frame rates exist for voice only traffic. However, such methods are insufficiently accurate and thus unsuitable for packet data traffic.
Therefore, there is a need in a variable rate communication system to accurately determine a transmitted data rate for packet data traffic at a receiver without embedding rate information into the transmitted data.
In the above described communication system, the mobile station sends signaling requests for SCCH assignment and de-assignment to the base station based on the amount of data the mobile station needs to transmit. In response, the base station dynamically allocates and de-allocates SCCHs via signaling messages. Assigning and de-assigning SCCHs via such signaling can be a relatively slow mechanism and thus wastes valuable data transmission bandwidth. For example, assigning or de-assigning an SCCH can take up to a half-second.
To reduce assignments and de-assignments and associated delays, a mobile station can operate in a discontinuous transmission (DTX) mode while a SCCH is assigned to the mobile station. The DTX mode permits the mobile station to stop transmitting on the assigned SCCH while data is unavailable. This is referred to as the DTX “black-out” period. The DTX mode also permits the mobile station to resume transmitting as soon as data becomes available, thus avoiding delays associated with assigning and de-assigning the SCCH. Transmitted data frames typically do not include DTX “on/off” information for similar reasons as mentioned above with regard to rate information. Since the receiver of the assigned SCCH receives no explicit indicator regarding the black-out periods, the receiver continuously demodulates and decodes the SCCH as long as the SCCH is assigned, even during the black-out period when no data is being transmitted, that is, when the demodulated and decoded data is invalid.
Therefore, it is desirable at a receiver in a communication system to discriminate between data transmission periods and black-outs so as to reduce a likelihood that invalid data is declared to be valid at the receiver.
In accordance with IS-95B, each transmitted SCCH data frame includes a 12 bit Cyclic Redundancy Code (CRC) for checking the validity of the data in the data frame at the receiver. Additional observable metrics, such as a Yamamoto measure, a symbol error rate, a frame energy, and so on, can be used to further improve on the CRC check. There is a finite probability (2−12=2.4×10−4) that demodulated random data associated with the black-out period, or noise corrupting a received data frame, will cause an erroneous match of the 12 bit CRC. In the case of a black-out period, a non-existent SCCH data frame or “random frame” corresponding to the erroneous CRC match, erroneously labels the invalid random frame as a valid data frame.
As is known, the transmitter and receiver typically implement complementary or parallel, layered, communication protocol layers including a physical protocol layer and an overlaying Radio Link Protocol (RLP) layer. One known RLP layer useable in wireless data communication stations is the IS-707 Radio Link Protocol. The physical layer sends (and receives) supposedly valid data frames (for example, data frames passing the CRC check as mentioned above) to (and from) the RLP. The RLP at the receiver tracks RLP frame sequence numbers embedded in the data frames for purposes of errored frames re-transmission and control.
During black out-periods, it has been observed that passing random frames as valid data frames to the RLP causes the RLP to initiate error control processes. This can occur on either the FCCH or SCCHs. For example, the RLP will reset and re-synchronize itself if the received sequence number, supposedly embedded in the random frame, is outside of a predetermined sequence number window (for example, 255) away from an expected sequence number. Alternatively, the RLP will request a retransmission of all of the data frames between the received and expected sequence numbers. In either case, the RLP error control processes disadvantageously reduce useful data throughput on the channel since most of the available bandwidth is utilized to re-sync the RLP or retransmit numerous data frames.
Therefore, there is a need to more accurately validate data frames at a receiver in a communication system, to thereby reduce the occurrence of such RLP error control processes and correspondingly increase channel bandwidth efficiency over conventional techniques.